Cylinders vs. Displacement

[squala]

Guys, educate me on this, please.

There are small displacement engines with more cylinders (e.g. 2-liter V6), and those with less cylinders but higher cc (like 2.5-liter inline 4).

In designing engines, which is generally better? Why? Thanks.

[mazdamazda]

generally, more cylinders will mean more hp & more importantly - torque (if the displacement is the same).

of course - this would also equate to higher cost of production.

[Bry]

I guess it all depends. Some 4 bangers have more power than some V6s. And some V6s can outgrunt some V8s. Take for example the Chevy Trailblazer's inline-6 vs. the Expedition's V8.

[mazdamazda]

I guess it all depends. Some 4 bangers have more power than some V6s. And some V6s can outgrunt some V8s. Take for example the Chevy Trailblazer's inline-6 vs. the Expedition's V8.

you also have to compare the power band & the peak power of the said vehicles.

for example:

Chevrolet Trailblazer 4.2 inline-6
275hp * 6000 rpm
373Nm * 3600 rpm

Ford Expedition 4.6 V8
232 * 4750 rpm
395Nm * 3450 rpm

the Exped's power is more accessible since the power peaks lower than the Trailblazer. though you have to see a dyno chart or test drive those vehicles to really see the difference. comparing in paper is sometimes futile.

[theveed]

I believe it's not power related directly. Having more cylinders allow a lower compression ratio to produce the same amount of power, normally the more cylinders you have, the smoother the engine will be, especially on higher speed since there's less friction and movement per cylinder.

Also, V configurations take up less space than inline counterparts. You can't have a short hood with a long inline engine.

The hp/torque values doesn't have to match a certain engine configuration at all.

[niky]

When it comes down to basics:

A 4-cylinder is cheap and easy to maintain. It can produce a lot of power if built to rev high, but in general, it can't produce a lot of power and torque at the same time, and if it does, those both come very high in the rev range.

A V6 can generally produce more torque and can rev higher and smoother than a 4-banger. It is favored for its smoothness, but costs more to make than a similar capacity I4.

An I6 has the same advantages as a V6, but is inherently unbalanced. Modern I6s, though, like BMWs, have found ways to balance out this resonance.

A V8 can produce more power, not as harmonious as a V6, though. Production-wise, cost is more than a V6 or I6 of similar capacity. American V8s are generally cheaper than comparable V6s and I6s ONLY because they're inherently simpler.

A V12 can rev smoother, produce more power, and rev much higher than a comparable V8. There's less resonance and it's easier to tune exhausts for V12s. Unfortunately, they're hellishly expensive and more complicated and heavier than V8s of the same capacity (given the same DISPLACEMENT and MATERIALS)

There are exceptions. Most cars would be running on small capacity V6s if they were cheaper.

A four-banger (I4) can be any size, from 1 liter up to 4 liters (old American cars, like the Jeep). The limit for practical 4-cylinders in street cars seems to be around 2.5. Modern large 4-cylinders need balancer shafts to allow them to rev high without shaking themselves (and the car) to pieces. At 2.0 liters and below, vibration isn't much of a problem.

Six Cylinders work well from 2.0 liters to 4.0 liters. Below that, they're just too complex to be worth it (Brabus has an experimental 1.3 V6 in its Smart Roadster, though), and above that, the cylinders would be just too big to rev high.

Eights and Twelves can be any size, limited by how complicated you want your engine and how much you're willing to spend. Ferrari has used 2 liter V12s in the past, and there's at least one motorbike with a V8. For common applications, 3.5 liters seems the basement level for a V8 or a V12. The upper limit seems to be decided on how fast you want the engines to run, but there are some ridiculously large V8s on supercars and racecars (7 liters and above) that seem to rev just fine.

[orly_andico]

i thought the I6 was inherently smoother than the V6 which is why BMW has been populating its cars with them for ages..

also i believe i read somewhere that the less cylinders you have, the faster the engine can spool up. hence the popularity of 4-cylinder high-revving engines for applications such as WRC where there's a lot of "up and down" speed as compared to NASCAR where you just go round and round at high speed.

niky, you also forgot the mutant W8, W12 and W16 engines that VW is so fond of

[jaeger]

Don't forget, there are also BOXER engines - like those in Subarus and Ferari's : flat 4's, flat 8's and flat 12's. They're like V-tpye engines with a 180-degree flat configuration. They're claimed to be the smoothest since the crank forces counter-oppose each other thus neutralizing engine vibration.

[squala]

Thanks for the lecture, peeps.

As for boxer engines, only Porsche and Subaru use this configuration, while the straight 6 layout is only employed by BMW as Lexus went back to the V platform in their new 6-cylinder engines. Generally, a V6 is the simplest to manufacture, and it can be applied to front-drive cars too. I'm pretty sure there are benefits from having the cylinders inline, thus BMW's better performance than its rivals.

[niky]

The I6 benefits from good torque transfer, I think, but I'm not sure. Any inline engine is inherently rougher than a V or a boxer (which is supposed to be the ultimate in power transfer, as none of the rotational inertia is wasted... well, the rotary is better, but we're talking pistons) but BMW has a way of perfectly balancing its I6s so that they can rev very high as compared to other I6s.

The I4 is the simplest to manufacture. WRC uses it because of the displacement restrictions... I4s at 2 liters are robust, sturdy and easy to turbocharge... and very easy to maintain as compared to engines with more cylinders at the same size.

V6s pack the most engine into the least space, that's why they're a favorite with car manufacturers. VW's VR6 (narrow angle) and V5 (!!!!) are extreme examples of this... as are the W-engines that are built based on the VR6 platform.

Boxer-6's are great, but they make maintenance and packaging difficult, which is why nobody else uses them.